For the last few years, federal investigators have been scoring dozens of major victories in insider-trading cases. They have sent one billionaire to prison and are on the verge of sending a former Goldman Sachs director there as well. They have effectively shut down one of the most successful hedge fund firms of all time.

Now, three more high-profile individuals are being probed by federal prosecutors, regulators and agents. A federal insider-trading probe is looking at Carl Icahn, the richest trader on Wall Street, Phil Mickelson, one of the nation’s most accomplished golfers, and William Walters, one of the nation’s most high-profile sports betters.

PGA Tour commissioner Tim Finchem and Phil Mickelson at the awards ceremony for The Players Championship in 2007, which Mickelson had just won. (Photo credit: Wikipedia)

But on the surface it seems like the investigation might not rise to the level of a federal prosecution or even a regulatory enforcement action. The investigation, which is reportedly being led by the Federal Bureau of Investigation and the Securities & Exchange Commission, has been going on for years. Icahn told news outlets on Friday that he did not know about any investigation and did not know Mickelson. Icahn called the suggestion that he was involved in improper trading “inflammatory and speculative.”

The investigation appears to be looking at whether Icahn leaked details of his failed takeover bid to Walters, who Icahn does know, and Mickelson. Both Walters and Mickelson reportedly traded in Clorox stock around the time of the takeover bid. As The New York Times pointed out yesterday, it would not be illegal for Icahn to leak details of a future trade and since he never joined Clorox’s board, he would not know any material confidential information about the company. Indeed, Clorox fought the veteran activist investor every step of the way. The New York Times did highlight that among the more aggressive legal theories that could be pursued is that Icahn breached a fiduciary duty to his own investors. But even that seems like a stretch given that at the time Icahn himself owned more than 90% of his Icahn Enterprises and had already moved to return all the outside money in his hedge fund.

Mickelson’s lawyer told The Wall Street Journal that the famous golfer was not a target of any investigation. Walters has not given any comment.